As described in U.S. Pat. Nos. 4,436,832 or 4,465,892 certain clays have an expandable network structure. They have the property of being able to adsorb especially water between the individual layers of which they are made up. This is true especially of the smectite group of clays. These clays have a structure which may be defined, in a simplified manner, as a three-sheet structure comprising two single sheets of SiO.sub.4 tetrahedrons and a dioctahedral or trioctahedral intermediate sheet of a hydrous metal oxide such as alumina or magnesia.
It is well known that the specific surface of a support is a very important factor in catalysis, and because of their property of being expandable, these clays might be used as catalysts or supports for catalysts, especially for the conversion of hydrocarbons. However, once expanded, in other words, after having adsorbed water between their individual layers, these clays have the drawback of losing their expanded character when heated to 100.degree. C., and consequently of not retaining the increase in specific surface which may result from their expansion.
It should be noted that the state of expansion of a clay is defined by the interlayer spacing and the basal spacing, which are measured by X-ray diffraction.
As its name implies, the interlayer spacing is the spacing between two layers. In the unexpanded state of the dried clay, it is zero.
The basal spacing, represented by the symbol d.sub.001, is defined as the sum of the thickness of a layer and the interlayer spacing.
In the case of montmorillonite, the thickness of a layer is 9.6 angstrom units. In the expanded state, the interlayer spacing may be as much as about 10 angstrom units, and the basal spacing may therefore be as much as about 20 angstrom units.
With a view to using expanding clays as catalyst supports or as catalysts, it has been sought to obtain clays having a maximum basal spacing that can be maintained when the clay is subjected to a heat treatment.
Thus, it has been found that pillars or bridges can be inserted between the clay layers to obtain pillared interlayered clays.
One well-known method consists in introducing between the clay layers, pillars formed by oligomers of a metal hydroxide, and in particular of an aluminum hydroxide, see U.S. Pat. No. 4,238,364.
In U.S. Pat. No. 4,452,910, a chromium oligomer is used. Other disclosures in this area are:
U.S. Pat. No. 4,176,090 PA1 U.S. Pat. No. 4,271,043 PA1 U.S. Pat. No. 4,248,739 PA1 U.S. Pat. No. 4,410,751 PA1 U.S. Pat. No. 4,216,188.
As explained in U.S. Pat. No. 4,452,910, in such methods the clay is treated with, e.g. hydroxy-aluminum polymers or oligomers in solution and the clay is dried and calcined to produce the supporting pillars between clay layers. These pillars maintain the expanded layer state in the clay and leave porosity framed by the pillars and the expanded layers. As reported in the patent, in smectite clays these resulting pores have a rectangular-type opening due to this framing by the pillars and clay layers.
Further, it is describe in U.S. Pat. No. 4,238,364 that interest in molecular sieve systems has been primarily concerned with the structure, properties, and application of natural and synthetic zeolites. Appropriately modified zeolites have gained importance as adsorbents in separation of industrial mixtures, or as catalysts for certain types of organic processes such as cracking, hydrocracking, isomerization, hydroisomerization, alkylation and dealkylation of simple aromatics, etc. However, it has been realized that there are certain severe limitations in the catalytic application of zeolites. In particular, due to the narrow range of critical pore sizes found in such systems (approximately 3-10A) intrasorption and reaction of bulky or even medium-sized organic molecules is impossible. For instance, it has been demonstrated that most of the molecules present in raw coal liquids cannot penetrate into the intracrystalline pores of conventional zeolite catalysts. Furthermore, certain organic substrates, including monocyclic aromatic compounds, have exhibited low intracrystalline diffusivity in zeolite media, resulting in poor recoveries and fast catalyst aging. Therefore, it is said to be highly desirable to prepare molecular sieve catalysts with expanded pore size which permit admission and free diffusion of large hydrocarbon and other molecules in the intracrystalline pore system. In the said patent, the oligomer is aluminum hydroxide and is used only in sufficient amount to partially cross-link or bridge the smectite layers so as to obtain lateral distances in the interlayer space of about 8.ANG. to 30.ANG. to increase pore size.
However, as will be shown in the ensuing description, a further effect in increasing pore size may be achieved by using another approach, viz., by markedly increasing the interlayer free spacing, also termed the gallery height, to hitherto unobtainable values for a pillared clay so that the products of the invention are highly suitable for use in catalytic processes involving the treatment of large molecules.
Of interest also is U.S. Pat. No. 4,241,035 which describes the synthesis of an inorganic material resembling the natural product, imogolite. See also U.S. Pat. No. 4,252,779.
U.S. Pat. No. 4,394,253 discloses a catalyst composition which includes two essential components: dispersed rods of fibrous form imogolite, and an inorganic oxide gel for bonding the rods. The inorganic oxide provides a rigid link between the imogolite rods, which are randomly oriented in a three-dimensional mutual orientation. The resulting rigid skeletal framework is said to provide a catalyst body with high crush strength and attrition resistance. A discussion of the inorganic oxide gels is given at col. 3, line 57 to col. 4, line 7. The patentee emphasizes that the imogolite rods are randomly (not systematically) oriented. Proper blending of the components is stated to be important to obtain the so described advantageous physical properties in the final catalyst. It is evident that the compositions are not pillared. See also U.S. Pat. No. 4,446,244.